Hello, bio people. Today we're going to discuss Griffiths and also Hershey and
Chase. Both of these experiments were very important in discovering exactly
how genetic material can be passed on from parent to offspring. Griffiths'
experiment happened first in 1928 and so we're going to discuss that one first.
Griffiths' experiment proved that bacteria are capable of passing on
genetic information.
Bacteria are capable of undergoing something called transformation; he thought that
information might be passed between organisms which might cause them to
assume a different form. Pneumonia was a major cause of death the time of
Griffith's experiment, and so he chose to experiment using the pneumonia bacterium.
Griffiths also knew there were two different strains of the pneumonia
bacteria; the rough strain or the "R strain" was relatively safe,
whereas the smooth strain or the "S strain" was pretty dangerous and almost
always killed the subject. Normally, anybody infected with the rough strain
would survive, anyone infected with the smooth strain would die.
Griffiths tried four different combinations; he first injected the R
strain into a healthy mouse, and the s strain into a healthy mouse. Predictably,
the mouse involved with the R strain survived, the mouse was injected with the
S strain died.
This was fairly predictable, but the next two steps show why Griffiths experiment
was groundbreaking. Remember this structure from the fall?
This structure that we drew way back when represented an enzyme, which is the
type of protein. Notice that it has a very specific shape. If the goal of this
enzyme is to put together these two substrates like this it, must have this
exact shape in order for the substrates to fit nicely. If this enzyme or protein
is subjected to any kind of heat, cold, acid, or force it will change its shape.
Now, even though this protein still has two openings for activation sites, the
substrates can no longer bind to it the way they should. Remember also that we
call this process denaturation. Griffiths hypothesized that the specific shape of
a bacterium would allow it to do something very similar;
using it's very specific shape, the bacteria would be able to bond other
substances inside the body.
However, if you expose it to heat, cold acid, or force, the substrates would no
longer be able to bind to the bacteria and the bacteria would not be able to
function normally.
Griffiths exposed his bacteria to a heat source. When exposed to the heat, the
bacterium changed their shape. Of course he wasn't able to see the bacteria
change its shape, but he was able to conclude that this is what had happened.
When he took the heat killed S strain, which used to be deadly, and injected it
into a mouse
amazingly enough the mouse survived. Heating the S strain bacteria had caused
them to become denatured, so they no longer were deadly. Griffiths then combined
heat killed S strain with live R strain. Remember R strain bacteria are
not considered to be deadly and heat killed as strain bacteria are not
considered to be deadly.
However when he combined the two of them and injected into the mouse, the mouse
died.
Griffiths hypothesized that even though the S strain bacteria had been denatured
by the heat, there were still components of it that were still active.
He thought that components of the S strain bacterium we're going into the R
strain bacterium and corrupting it somehow. This could cause what was
previously innocuous R strain to become a toxic R strain. We call this
"transformation".
Now let's talk about Hershey and Chase. At the time, most people thought the
molecule of heredity was contained within proteins, almost no one believed
that the molecule heredity was nucleic acids. Proteins are so complex that it
wasn't that big of a stretch to imagine that there might be a nose shape protein,
or a height protein, or personality set of proteins. There are approximately a
hundred and fifty thousand different proteins in the human body. Remember that
proteins are made up of 20 different amino acids which are arranged in very
complicated patterns and shapes.
This leads to a huge number of potential combinations. Nucleic acids are much
simpler. There are only four different kinds of nucleotides and there are only
a few different types of nucleic acids; DNA and a few different types of RNA.
Hershey and Chase set out to prove which of these two things, proteins or nucleic
acids, was responsible for passing along genetic material. They chose to do their
work using bacteriophages. Bacteriophages have several different advantages,
including the fact that they reproduce very quickly and are only made up of two
of the "Big Four"; nucleic acids and proteins. They don't contain any lipids
or carbohydrates. So here we can see that this bacteriophage has a protein coat on
the outside, and the nucleic acids on the inside. Bacteriophages reproduce by
attaching to the outside of a host cell. Then they inject their genetic material
into it. The protein coat stays on the outside of the host cell. The DNA from the
bacteriophage then causes the cell to stop whatever it was doing and begin
producing copies of the bacteriophage. The new bacteriophages are identical to
the parent bacteriophage that originally infected the cell. The host
cell then explodes, releasing all of the new bacteriophages, which then go out and
infect even more cells. Hershey and Chase used this life cycle to prove which
of the two contenders was actually responsible for inheritance; nucleic
acids or proteins. Hershey and Chase used two groups of bacteriophages. In one
group, they chose to test the protein coat, and the other group they chose to
test the nucleic acids.
To test the two options, they decided to make parts of each type of bacteriophage
radioactive. They knew that whichever part was responsible for genetic
inheritance would create radioactive baby bacteriophages. For the protein coat,
they made the protein coat radioactive. Since proteins contain the elements
sulfur, they decided to use a radioactive type of sulfur. The type of sulfur they
use is called sulfur 35. Since sulfur is not found in nucleic acids, they were
only marking the protein coat for the nucleic acid group. They decided to
replace the naturally-occurring phosphorus in the nucleic acids with a
radioactive type of phosphorus called phosphorus 32. Since there was no
phosphorus found in the protein coat, the only thing they were marking in the
second group was the nucleic acids. Hershey and Chase then allowed each type
of bacteriophage to infect a host cell. They knew that whichever type was
responsible for inheritance would create radioactive bacteriophages. The
bacteriophages then injected their genetic material into the two host cells.
Remember the protein coat is left on the outside of the host cell, and only the
DNA gets injected. Hershey and chase then shook the compounds in a blender to
shake the protein coats off the outside of the cells. The protein coats then
dissolved in the liquid portion of the experiment, which is called the
supernatant. So, now each blender has a liquid portion which contains the
dissolved protein coats, and a solid portion that contains everything else.
However at the moment they are mixed together in the blender and so we need
separate them. To separate the liquid from the solids, the scientists then put
the mixture into a centrifuge which spins very fast and separates components
based on density. After spinning the components in the centrifuge, we now have
a liquid portion containing the protein coats and the solid portion containing
everything else. We call the solid portion the pellet.
Hershey and Chase then tested both the pellet and the supernatant in each case
to determine which of them was radioactive. Remember the goal was to
make radioactive bacteriophages which would be found in the pellet. In the
situation where they had tagged the protein coat, the supernatant was
radioactive. However, the pellet containing the new bacteriophages was
not radioactive and so they decided that must not be the molecule of
inheritance. In the case of the radioactive nucleic acids however, all of
the new bacteriophages contained within the pellet were radioactive and this
told them that nucleic acid was actually the molecule of inheritance.